In recent years much time and effort have been spent on the research and development of methods for recovering fuel from oil shale. Oil shale contains kerogen, a complex hydrocarbon substance intercalated in thin layers throughout the shale. Through the application of heat, kerogen can be destructively distilled to yield a liquid, commonly called shale oil but, herein referred to alternatively by a new term, "keronol"; the distillation also yields some gaseous products. In order to assure efficient heating, the oil shale is broken into fragments sufficiently small to permit the heat to saturate the shale and to allow the produced shale oil or keronol and gases to escape from the mass of fragments. The conventional mining of oil shale, followed by crushing and retorting above ground is effective to produce shale oil and gas, however, this procedure leaves large quantities of spent shale or barren rock at the surface, the disposition of which involves an environmental problem. For the purpose of avoiding the large scale accumulation of spent shale, various methods have been proposed and tested for the retorting of the oil shale underground or in situ so that the quantity of oil shale which must be mined out is minimized. Calculations and experience indicate that the total aggregate volume of the shale which must be mined out of an in situ retorting zone is of the order of fifteen to twenty percent of the entire mass before fragmenting. Various methods have been proposed for providing voids in the oil shale deposit and fragmenting or "rubbleizing" the shale to fill the voids with the fragmented shale and provide the permeability of the mass of shale required for effective retorting. The fragmented oil shale is ignited at, or near the top of the prepared retort, and combustion is maintained by injecting oxygenated gases into the retort. Retorting can also be done without combustion by circulating hot, inert gases through the rubble. Heat generated by the combustion decomposes the kerogen in the shale to produce liquid and gaseous products and these flow downwardly through the fragmented mass and are removed.
Much of the attention of those involved in the developing oil shale industry is now directed toward the retorting of the oil shale underground. Underground retorting is complicated and an expensive operation and in order that such operations be commercially feasible it is necessary that the retorting of very large deposits be carefully planned; such retorting may, for example, require a multiplicity of separate retort zones, each of which may be capable of yielding a million or more barrels of shale oil. Such deposits may extend for many square miles and may range up to several hundred feet in depth or thickness. Tracts of land containing such deposits may, for example, comprise a multiplicity of separate retorts, in each of which the retorting operation may be carried or independently of the others and the retorts may be operated in a planned sequence to provide a continuous supply of the keronol product. The preparation and operation of such planned retorting of an oil shale deposit requires many engineers and technicians and a large number of workmen, and it is essential, for efficient production of shale oil, that the working areas be arranged for effective and efficient preparation and operation of the retorts and it is desirable that reliable and easily installed and monitored instruments be provided for controlling the retorting operation. It is also essential that the preparation and operation of the retorts be effected with maximum safety for the personnel.
The present invention is directed particularly toward providing a highly effective and efficient procedure for preparing the oil shale deposit for underground retorting and for providing safe working zones and conditions for all personnel.
It is an object of this invention to provide an improved method for preparing an underground oil shale deposit for in situ retorting.
It is another object of this invention to provide an improved method for preparing an underground oil shale depsoit for in situ retorting and for assuring safe operating conditions both during the preparation of the retort and during its operation.
It is another object of this invention to provide a method for preparing an underground oil shale deposit for in situ retorting including an improved method for providing and installing devices for effecting the monitoring and control of the retorting operation.
It is a further object of this invention to provide an improved method for preparing an underground oil shale deposit for in situ retorting wherein a multiplicity of contiguous retorts are prepared and including an improved arrangement for assuring the effective and safe operation of the contiguous retorts.
It is a further object of this invention to provide a process for utilizing heat energy remaining in a depleted retort after completion of the retorting operation.
It is a further object of this invention to provide a process for utilizing the heat energy in a depleted retort for the recovery of additional product.
It is a still further object of this invention to provide a method for utilizing the spent shale from surface retorting operations for filling the voids in the fragmented shale in depleted retorts.
Briefly, in carrying out the objects of this invention in one embodiment thereof, an oil shale deposit is prepared for retorting by excavating a working and fluid collecting zone in the formation below the deposit and excavating a predetermined volume of shale from the retort zone to form voids therein in a preselected configuration and sufficient in volume to contain the increased volume of the shale after fragmentation in the retort, preparing a drilling and process control level above the retort and separated from the roof of the retort zone by a substantial depth of overburden and drilling holes into the shale between the voids to form a pattern of holes for receiving explosive charges and setting off the charges to fragment the shale and fill the voids with the fragmented shale. Before fragmenting the shale, certain of the drill holes are cased above the retort zone and the remaining holes are plugged. The fragmented shale is then ignited and selected ones of the cased holes are used for facilitating the monitoring and control of the retorting of the fragmented shale and the fluid products resulting from retorting are directed downwardly into the collecting zone. The preparation of the drilling and process control level and of the collecting and pumping zone provides safe working areas isolated from the retorting zone and enabling the workers to monitor and control the retorting operation in safe locations.